Bidirectional dc-dc converter and control method

Abstract

This application relates to a bidirectional DC-DC converter and its control method, including an input inductor, a primary-side full-bridge circuit, a high-frequency transformer, a secondary-side half-bridge circuit, an output capacitor unit, a mode switching switch, and a control circuit. The control circuit controls the on / off state of the mode switching switch and the operating states of six switches based on the detected input voltage or state of charge of the flow battery, enabling the bidirectional DC-DC converter to switch between multiple modes: at low voltage, the input inductor first boosts the voltage, and the high-frequency transformer then boosts it again to form a composite gain, avoiding efficiency degradation caused by the limiting duty cycle; at high voltage, the mode switching switch bypasses the high-frequency transformer, and energy is directly transferred to the DC bus, reducing reactive circulating current; the input inductor reduces input current ripple, which helps reduce polarization losses in the flow battery. This improves the converter's operating efficiency under wide voltage conditions of the flow battery and enables efficient bidirectional energy transfer over a wide input range.

Description

Technical Field

[0001] This application relates to the field of piezoelectric conversion technology, and in particular to a bidirectional DC-DC converter and its control method. Background Technology

[0002] Flow batteries (such as vanadium redox flow batteries) are a type of electrochemical energy storage device that stores and releases energy through redox reactions of active materials in an electrolyte. Compared to traditional lithium-ion batteries, flow batteries have advantages such as capacity and power decoupling, long cycle life, high intrinsic safety, and suitability for large-scale energy storage. In recent years, they have received widespread attention in areas such as renewable energy grid connection, grid peak shaving and frequency regulation, and long-term energy storage.

[0003] However, from the perspective of power electronic interfaces, flow batteries exhibit a series of electrical characteristics that differ from traditional energy storage units, mainly in the following aspects: First, the terminal voltage of a flow battery varies significantly with its state of charge (SOC). In actual operation, its output voltage typically varies over a wide range (e.g., 0.5 to 1.5 times or even more of the rated voltage), which places dual demands on the DC-DC converter for a wide input voltage range and high voltage gain capability.

[0004] Secondly, flow battery systems require frequent charge-discharge switching, so the interface converter must have stable and reliable bidirectional energy transfer capabilities and maintain high efficiency and good dynamic response performance during forward and reverse operation.

[0005] Furthermore, because the electrochemical reaction process in flow batteries is highly sensitive to current fluctuations, large current ripple can exacerbate electrolyte concentration polarization, leading to decreased energy efficiency and shortened battery life. Therefore, the interface converter is required to have low current ripple characteristics and good current regulation capability.

[0006] In addition, flow battery energy storage systems are typically connected in medium to high voltage DC mode (hundreds to kilovolt levels), which places higher demands on the electrical isolation capability of the converter, device voltage stress control, and system safety.

[0007] Based on the above characteristics, a bidirectional DC-DC converter is typically required between the flow battery and the DC bus to achieve voltage matching, power regulation, and bidirectional energy flow. In existing technologies, dual active bridge converters are commonly used. A dual active bridge converter is a typical high-frequency isolated bidirectional DC-DC converter. Its basic structure consists of primary and secondary full-bridge circuits coupled through a high-frequency transformer. Power transfer control is achieved by adjusting the phase shift angle between the two bridges. Theoretically, dual active bridge converters have good symmetry and bidirectional power flow capability. Their power transfer relationship can usually be approximated as a function related to the phase shift angle and the voltage difference between the two sides, thus making them widely used in medium-to-high power applications. However, in wide-voltage range applications of flow batteries, the voltage matching capability of dual active bridge converters depends on the transformer turns ratio and phase shift angle adjustment. When the input-output voltage ratio deviates from the design point, its transferable power decreases significantly, making it difficult to cover the full SOC range of the flow battery. At non-optimal operating points, a large reactive circulating current exists inside the dual active bridge converter. This circulating power does not participate in effective energy transfer but significantly increases conduction losses and reduces system efficiency. Therefore, traditional dual active bridge converters are difficult to achieve efficient operation under all operating conditions in the wide voltage range of flow batteries.

[0008] Therefore, how to improve the operating efficiency of the converter under the wide voltage operating conditions of the flow battery is a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0009] This application provides a bidirectional DC-DC converter and control method to improve the converter's operating efficiency under wide voltage operating conditions of flow batteries.

[0010] To achieve the above objectives, the present invention provides the following technical solution: A bidirectional DC-DC converter includes an input inductor, a primary-side full-bridge circuit, a high-frequency transformer, a secondary-side half-bridge circuit, an output capacitor unit, a mode switching switch, and a control circuit, wherein: The first end of the input inductor is used to connect to the positive terminal of the flow battery; The primary-side full-bridge circuit consists of a first switch, a second switch, a third switch, and a fourth switch. The first switch and the second switch are connected in series to form a first bridge arm. The third switch and the fourth switch are connected in series to form a second bridge arm. The first bridge arm and the second bridge arm are connected in parallel. The primary-side full-bridge circuit has a positive input terminal and a negative input terminal. The negative input terminal of the primary-side full-bridge circuit is used to connect to the negative electrode of the flow battery. The two ends of the primary winding of the high-frequency transformer are respectively connected to the midpoint of the first bridge arm and the midpoint of the second bridge arm. The secondary half-bridge circuit consists of a fifth switch and a sixth switch connected in series; The output capacitor unit consists of a first capacitor and a second capacitor connected in series, and the two ends of the output capacitor unit are used to connect to the DC bus. The first terminal of the mode switching switch is connected to the second terminal of the input inductor, the second terminal of the mode switching switch is connected to the positive terminal of the output capacitor unit, and the positive input terminal of the primary-side full-bridge circuit is connected to the connection node between the input inductor and the mode switching switch. The secondary half-bridge circuit is connected in parallel with the output capacitor unit. The first end of the secondary winding of the high-frequency transformer is connected to the connection node of the fifth switch and the sixth switch, and the second end of the secondary winding is connected to the connection node of the first capacitor and the second capacitor. The control circuit is connected to the primary-side full-bridge circuit, the secondary-side half-bridge circuit, and the mode switching switch, respectively, and is used to control the on / off state of the mode switching switch and the working state of the first switch, the second switch, the third switch, the fourth switch, the fifth switch, and the sixth switch according to the input voltage or state of charge of the flow battery.

[0011] Optionally, in the above-described bidirectional DC-DC converter, the control circuit is configured as follows: When the input voltage of the flow battery is lower than the first preset threshold, the mode switching switch is turned off, and the first and fourth switches are alternately turned on at high frequency in conjunction with the second and third switches. The bidirectional DC-DC converter operates in the enhanced boost discharge mode. The input inductor and the high-frequency transformer together form a composite boost path. Energy is boosted by the flow battery through the input inductor and then boosted by the high-frequency transformer to the DC bus.

[0012] Optionally, in the above-described bidirectional DC-DC converter, the control circuit is further configured as follows: When the input voltage of the flow battery is higher than the second preset threshold, the mode switching switch is controlled to turn off or to be synchronously rectified and turned on. The first switch, the second switch, the third switch and the fourth switch are controlled to be turned on or off at the same time, so that the bidirectional DC-DC converter operates in the direct boost discharge mode. The input inductor is a boost energy storage element. The energy is boosted by the flow battery through the input inductor and then transmitted to the DC bus through the mode switching switch.

[0013] Optionally, in the above-described bidirectional DC-DC converter, in the enhanced boost discharge mode, the voltage gain of the bidirectional DC-DC converter is made to satisfy the following by adjusting the first duty cycle of the first switch, the second switch, the third switch, and the fourth switch: Where n is the turns ratio of the high-frequency transformer. This is the first duty cycle; In the direct boost discharge mode, by adjusting the second duty cycle of the first switch, the second switch, the third switch, and the fourth switch, the voltage gain of the converter is made to satisfy... ,in This is the second duty cycle.

[0014] Optionally, in the above-described bidirectional DC-DC converter, the control circuit is configured as follows: When the DC bus charges the flow battery and the input voltage of the flow battery is lower than the third preset threshold, the fifth and sixth switches are controlled to conduct alternately at high frequency, and the first, second, third, and fourth switches are conducted in a synchronous rectification mode, so that the converter operates in the enhanced step-down charging mode. The energy is stepped down by the high-frequency transformer from the DC bus and then transferred to the flow battery through the input inductor.

[0015] Optionally, in the above-described bidirectional DC-DC converter, the control circuit is further configured as follows: When the input voltage of the flow battery is higher than the fourth preset threshold, the mode switching switch is controlled to operate at high frequency, and the first switch, the second switch, the third switch and the fourth switch are controlled to be turned on or off at the same time, so that the bidirectional DC-DC converter operates in direct buck charging mode, and energy is transferred from the DC bus to the flow battery through the mode switching switch and the input inductor.

[0016] Optionally, in the above-described bidirectional DC-DC converter, in the enhanced buck charging mode, the voltage gain of the bidirectional DC-DC converter is made to satisfy the following by adjusting the third duty cycle of the fifth and sixth switches: Where n is the turns ratio of the high-frequency transformer. This refers to the third duty cycle; In the direct buck charging mode, by adjusting the fourth duty cycle of the mode switching switch, the voltage gain of the bidirectional DC-DC converter is made to meet the requirements. ,in This refers to the fourth duty cycle.

[0017] The bidirectional DC-DC converter provided by this invention has the following configuration: First end of the input inductor is connected to the positive terminal of the flow battery, and second end is connected to the first end of the mode switching switch and the positive input terminal of the primary-side full-bridge circuit. The negative input terminal of the primary-side full-bridge circuit is connected to the negative terminal of the flow battery, and the midpoints of its two bridge arms are connected to the two ends of the primary winding of the high-frequency transformer. One end of the secondary winding of the high-frequency transformer is connected to the connection node of the fifth and sixth switches, and the other end is connected to the connection node of the first and second capacitors. The secondary half-bridge circuit and the output capacitor unit are connected in parallel, and their two ends are connected to the DC bus. The control circuit controls the on / off state of the mode switching switch and the operating state of the six switches based on the detected input voltage or state of charge of the flow battery, enabling the bidirectional DC-DC converter to switch between multiple modes: at low voltage, the input inductor first boosts the voltage, and the high-frequency transformer boosts it again to form a composite gain, avoiding efficiency degradation caused by the extreme duty cycle; at high voltage, the mode switching switch bypasses the high-frequency transformer, and energy is directly transferred to the DC bus, reducing reactive circulating current; simultaneously, the input inductor reduces input current ripple, which helps reduce polarization losses of the flow battery. Therefore, the bidirectional DC-DC converter improves the converter's operating efficiency under wide voltage conditions of flow batteries, and can achieve efficient bidirectional energy transfer within a wide input range of 10V to 200V, solving the problems of low efficiency and large ripple in existing technologies.

[0018] This application also provides a control method for using the bidirectional DC-DC converter as described in any of the above embodiments, including: The input voltage or state of charge of the flow battery is detected by the control circuit. Based on the detection results of the control circuit, the on / off state of the control mode switching switch and the working states of the first, second, third, fourth, fifth, and sixth switches are controlled to switch the working mode of the bidirectional DC-DC converter.

[0019] Optionally, in the above control method, the operating modes include enhanced boost discharge mode, enhanced buck charge mode, direct boost discharge mode, and direct buck charge mode; The enhanced boost discharge mode and the direct boost discharge mode are operating modes in the discharge direction, while the enhanced buck charging mode and the direct buck charging mode are operating modes in the charging direction.

[0020] Optionally, in the above control method, When the bidirectional DC-DC converter operates in the enhanced boost discharge mode, the mode switching switch is turned off, and the first switch and the fourth switch are switched on alternately with the second switch and the third switch at complementary high frequencies, so that the input inductor and the high-frequency transformer form a composite boost path. When the bidirectional DC-DC converter operates in the direct boost discharge mode, the mode switching switch is controlled to turn off or to be synchronously rectified and turned on, and the first switch, the second switch, the third switch and the fourth switch are controlled to be turned on or off at the same time, so that the input inductor is a boost energy storage element, and the mode switching switch provides a direct path from the input inductor to the DC bus. When the bidirectional DC-DC converter operates in the enhanced buck charging mode, the fifth switch and the sixth switch are controlled to conduct alternately, and the first switch, the second switch, the third switch and the fourth switch are controlled to conduct in a synchronous rectification manner, so that the DC bus energy is stepped down by the high-frequency transformer and then transmitted to the flow battery through the input inductor; When the bidirectional DC-DC converter operates in the direct buck charging mode, the high-frequency switch of the mode switching switch is controlled to operate, and the first switch, the second switch, the third switch and the fourth switch are controlled to be turned on or off at the same time, so that the DC bus energy is directly transmitted to the flow battery through the mode switching switch and the input inductor.

[0021] The specific structure of the bidirectional DC-DC converter is as described in the above embodiments. Since this control method adopts all the technical solutions of all the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be described in detail here. Attached Figure Description

[0022] The accompanying drawings, incorporated in and forming part of this specification, illustrate embodiments consistent with this application and, together with the description, serve to explain the principles of this application. To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, those skilled in the art can obtain other drawings based on these drawings without creative effort. One or more embodiments are illustrated by way of example through the corresponding images in the accompanying drawings. These exemplary descriptions do not constitute a limitation on the embodiments. Elements with the same reference numerals in the drawings represent similar elements. Unless otherwise stated, the figures in the drawings do not constitute a limitation on scale.

[0023] Figure 1 A circuit diagram of stage 1-A of the enhanced boost discharge mode provided in the embodiments of this application; Figure 2 A circuit diagram of stage 1-B of the enhanced boost discharge mode provided in the embodiments of this application; Figure 3 A circuit diagram of stage 1-C of the enhanced boost discharge mode provided in the embodiments of this application; Figure 4 A circuit diagram of stage 3-A of the direct boost discharge mode provided in an embodiment of this application; Figure 5 A circuit diagram of stage 3-B of the direct boost discharge mode provided in an embodiment of this application; Figure 6 A circuit diagram of stage 2-A of the enhanced buck charging mode provided in the embodiments of this application; Figure 7 A circuit diagram of stage 2-B of the enhanced buck charging mode provided in an embodiment of this application; Figure 8 A circuit diagram of stage 4-A of the direct buck charging mode provided in the embodiments of this application; Figure 9 The circuit diagram for stage 4-B of the direct buck charging mode provided in the embodiments of this application. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of this application clearer, the technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0025] The following disclosure provides numerous different embodiments or examples for implementing various structures of this application. To simplify the disclosure, specific examples of components and arrangements are described below. These are merely examples and are not intended to limit the scope of this application. Furthermore, reference numerals and / or letters may be repeated in different examples. Such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed. In addition, various specific examples of processes and materials are provided in this application; however, those skilled in the art will recognize the applicability of other processes and / or the use of other materials.

[0026] For ease of description, spatial relative terms may be used in the text to describe the relative position or movement of one element or feature relative to another element or feature, as shown in the figure. These relative terms include, for example, "inside," "outside," "middle," "outer," "below," "below," "above," "front," "back," etc. Such spatial relative terms are intended to include different orientations of the device in use or operation, other than those depicted in the figure. For example, if the device in the figure undergoes a positional flip, orientation change, or change of motion, these directional indications will change accordingly. For instance, an element described as "below other elements or features" or "below other elements or features" will subsequently be oriented "above other elements or features" or "above other elements or features." Therefore, the example term "below" can include both upper and lower orientations. The device may be otherwise oriented (rotated 90 degrees or in other directions), and the spatial relative descriptors used in the text will be interpreted accordingly.

[0027] This application provides a bidirectional DC-DC converter, including an input inductor L1, a primary-side full-bridge circuit, a high-frequency transformer T, a secondary-side half-bridge circuit, an output capacitor unit, a mode switching switch M, and a control circuit. The first end of the input inductor L1 is connected to the positive terminal of a flow battery. The primary-side full-bridge circuit consists of a first switch S1, a second switch S2, a third switch S3, and a fourth switch S4. The first switch S1 and the second switch S2 are connected in series to form a first bridge arm, and the third switch S3 and the fourth switch S4 are connected in series to form a second bridge arm. The first and second bridge arms are connected in parallel. The primary-side full-bridge circuit has a positive input terminal and a negative input terminal. The negative input terminal of the primary-side full-bridge circuit is connected to the negative terminal of the flow battery. The two ends of the primary winding of the high-frequency transformer T are respectively connected to the midpoints of the first and second bridge arms. The secondary-side half-bridge circuit consists of a fifth switch Q1 and a sixth switch Q2 connected in series. The output capacitor unit consists of a first switch S1 connected in series with a second switch S2. The output capacitor unit consists of capacitor C1 and capacitor C2, with both ends connected to the DC bus. The first end of the mode switching switch M is connected to the second end of the input inductor L1, and the second end of the mode switching switch M is connected to the positive terminal of the output capacitor unit. The positive input terminal of the primary full-bridge circuit is connected to the connection node between the input inductor L1 and the mode switching switch M. The secondary half-bridge circuit is connected in parallel with the output capacitor unit. The first end of the secondary winding of the high-frequency transformer T is connected to the connection node between the fifth switch Q1 and the sixth switch Q2, and the second end of the secondary winding is connected to the connection node between the first capacitor C1 and the second capacitor C2. The control circuit is connected to the primary full-bridge circuit, the secondary half-bridge circuit, and the mode switching switch M, respectively, and is used to control the on / off state of the mode switching switch M and the working state of the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch Q1, and the sixth switch Q2 according to the input voltage or state of charge of the flow battery.

[0028] Specifically, the input inductor L1 can serve as a filter inductor, ensuring a smooth and continuous input current and reducing current ripple on the flow battery side. It can also function as a boost inductor, participating in energy conversion and boosting voltage under specific operating modes. The primary-side full-bridge circuit inverts the DC power from the flow battery into a high-frequency AC square wave, achieving voltage transformation and power flow control through the switching timing. The secondary-side half-bridge circuit can function as a synchronous rectifier to convert high-frequency AC to DC, or as an inverter to reverse the DC bus energy to the primary-side full-bridge circuit.

[0029] Specifically, the high-frequency transformer T is used to achieve electrical isolation and voltage matching. The primary winding of the high-frequency transformer T receives high-frequency AC power from the primary full-bridge circuit, while the secondary winding outputs an induced voltage. The turns ratio n (primary turns / secondary turns) of the high-frequency transformer T can be designed according to system requirements. For example, if a wide range of input voltage (10V to 200V) and output voltage (800V) is required, the turns ratio n can be between 1:4 and 1:8, which will not be elaborated further here.

[0030] The bidirectional DC-DC converter provided by this invention has the following configuration: First end of input inductor L1 is connected to the positive terminal of a flow battery; second end is connected to the first terminal of mode switching switch M and the positive input terminal of the primary-side full-bridge circuit. The negative input terminal of the primary-side full-bridge circuit is connected to the negative terminal of the flow battery, and the midpoints of its two bridge arms are connected to the two ends of the primary winding of high-frequency transformer T. One end of the secondary winding of high-frequency transformer T is connected to the connection node of the fifth switch Q1 and the sixth switch Q2, and the other end is connected to the connection node of the first capacitor C1 and the second capacitor C2. The secondary half-bridge circuit and the output capacitor unit are connected in parallel, and their two ends are connected to the DC bus. The control circuit, based on the detected input voltage or state of charge of the flow battery, controls the on / off state of the mode switching switch M and the operating states of the six switches, enabling the bidirectional DC-DC converter to switch between multiple modes: at low voltage, the input inductor L1 first boosts the voltage, and the high-frequency transformer T boosts it again to form a composite gain, avoiding efficiency degradation caused by the extreme duty cycle; at high voltage, the mode switching switch M bypasses the high-frequency transformer T, and energy is directly transferred to the DC bus, reducing reactive circulating current; at the same time, the setting of the input inductor L1 reduces the input current ripple, which helps to reduce the polarization loss of the flow battery. Therefore, the bidirectional DC-DC converter improves the converter's operating efficiency under wide voltage conditions of the flow battery, achieving efficient bidirectional energy transfer within a wide input range of 10V to 200V, solving the problems of low efficiency and large ripple in existing technologies.

[0031] To optimize the above technical solution, the control circuit is configured such that when the input voltage of the flow battery is lower than the first preset threshold, the control mode switching switch M is turned off, and the first switch S1 and the fourth switch S4 are controlled to conduct alternately at high frequency with the second switch S2 and the third switch S3. The bidirectional DC-DC converter operates in the enhanced boost discharge mode, and the input inductor L1 and the high-frequency transformer T together form a composite boost path. The energy is boosted by the flow battery through the input inductor L1 and then boosted by the high-frequency transformer T to be transmitted to the DC bus.

[0032] Specifically, see Figures 1-3 The high-frequency switching cycle of the enhanced boost discharge mode consists of three stages: Phase 1-A: Switches S1, S2, S3, and S4 are all turned on. The input voltage is applied to the input inductor L1, causing the current in L1 to rise linearly, increasing the energy stored in L1. The primary winding of the high-frequency transformer T is short-circuited by two parallel paths formed by switches S1 and S4, and S2 and S3, resulting in zero primary voltage and no energy transfer to the secondary winding. Switches Q1 and Q2 are both turned off. The duty cycle for this phase is D1, where D1 > 0.5, and the input inductor L1 is charged.

[0033] Phase 1-B: Switches S2 and S3 are off, while switches S1 and S4 remain on. The current path is: input inductor L1 → switch S1 → primary winding of high-frequency transformer T → switch S4. Switch Q1 is on (synchronous rectification), and switch Q2 is off. The current path is: after passing through capacitor C2, it splits into two branches: the first branch flows through capacitor C1 to the positive terminal of the DC bus; the second branch flows through the secondary winding of high-frequency transformer T and switch Q1 (on) to the positive terminal of the DC bus. Simultaneously, input inductor L1 continues to release energy to high-frequency transformer T, and the inductor current decreases linearly.

[0034] Phase 1-C: Switches S1 and S4 are off, while switches S2 and S3 are on. The current path is: input inductor L1 → switch S3 → primary winding of high-frequency transformer T → switch S2. Switch Q2 is on, and switch Q1 is off. The current path is divided into two branches: the first branch includes the second capacitor C2, and the second branch includes the sixth switch Q2 and the secondary winding. After the two branches merge, the current flows out through the first capacitor C1, and the input inductor L1 continues to discharge.

[0035] In the above process, the input inductor L1 first stores energy, then transfers the energy to the high-frequency transformer T. The high-frequency transformer T then boosts the voltage before sending it to the output. The voltage gain is... Where n is the duty cycle, the first duty cycle This refers to the duty cycle of the first switch S1 and the fourth switch S4 (which is also equal to the duty cycle of the second switch S2 and the third switch S3). In some embodiments, >0.5, that is, 1 / (1- The gain can be greater than 2, and with the high-frequency transformer T and its turns ratio n, the total gain can reach tens of times, thus meeting the requirement of boosting a 10V input to 800V. The enhanced boost-discharge mode avoids the problems of decreased control sensitivity and increased switching losses in traditional Boost converters at extremely high duty cycles through a composite gain path of "input inductor L1 boosting first, high-frequency transformer T boosting second." Simultaneously, the continuous current of the input inductor L1 and low ripple are beneficial to the chemical stability of the flow battery. Synchronous rectification in the secondary half-bridge circuit reduces conduction losses.

[0036] To optimize the above technical solution, the control circuit is further configured to: when the input voltage of the flow battery is higher than the second preset threshold, the control mode switching switch M is turned off or its synchronous rectification is turned on, and the first switch S1, the second switch S2, the third switch S3 and the fourth switch S4 are simultaneously turned on or off, so that the bidirectional DC-DC converter operates in the direct boost discharge mode. The input inductor L1 is a boost energy storage element. The energy is boosted by the flow battery through the input inductor L1 and then transmitted to the DC bus through the mode switching switch M.

[0037] Specifically, the control circuit executes: turning off the mode switching switch M or putting the mode switching switch M into synchronous rectification mode; and simultaneously turning on or off the first switch S1, the second switch S2, the third switch S3, and the fourth switch S4 of the primary-side full-bridge circuit, i.e., the four switches operate synchronously, with the duty cycle denoted as . (In this embodiment, typically) The value is between 0.6 and 0.8. At this time, the primary-side full-bridge no longer functions as an inverter. Instead, when the first switch S1, the second switch S2, the third switch S3, and the fourth switch S4 are simultaneously turned on, the lower end of the input inductor L1 is shorted to Vin-; when the first switch S1, the second switch S2, the third switch S3, and the fourth switch S4 are simultaneously turned off, the lower end of the input inductor L1 is connected to the output positive terminal through the mode switching switch M. The fifth switch Q1 and the sixth switch Q2 of the secondary-side half-bridge remain off.

[0038] Specifically, see Figure 4 and Figure 5 The direct boost discharge mode consists of two phases within one switching cycle: Phase 3-A: Switches S1, S2, S3, and S4 are all on, while switches Q1 and Q2 are both off, and mode switching switch M is off. Two current loops exist in this phase: Circuit 1 (Primary-side energy storage circuit): Flow battery positive terminal → Input inductor L1 → First switch S1, Second switch S2, Third switch S3, Fourth switch S4 → Flow battery negative terminal. Input inductor L1 stores energy, and the current increases linearly.

[0039] Circuit Two (Output Capacitor Filtering Circuit): DC bus negative terminal → second capacitor C2 → first capacitor C1 → DC bus positive terminal. This circuit maintains the voltage balance of the output capacitor unit and provides a smooth DC voltage.

[0040] During this stage, the primary winding of the high-frequency transformer T is short-circuited by four conducting switches, the primary voltage is zero, there is no induced voltage on the secondary side, and the transformer does not participate in energy transfer. Energy is stored.

[0041] Phase 3-B: Switches S1, S2, S3, and S4 are all off; mode switching switch M is on (synchronous rectification); switches Q1 and Q2 remain off. Two current loops exist in this phase: Circuit 1 (Battery Discharge Main Circuit): Flow battery positive terminal → Input inductor L1 → Mode switch M → First capacitor C1 → Second capacitor C2 → Flow battery negative terminal. Input inductor L1 releases energy, the current decreases linearly, charging the first capacitor C1 and the second capacitor C2, increasing the output voltage.

[0042] Circuit 2 (Output Capacitor Filtering Circuit): DC bus negative terminal → second capacitor C2 → first capacitor C1 → DC bus positive terminal. This circuit maintains the voltage balance of the output capacitor unit, and is the same as Circuit 2 in Stage 3-A.

[0043] During this stage, the high-frequency transformer T is in an open-circuit state because all primary-side switches are turned off, and it does not participate in energy transmission. Since the high-frequency transformer T is bypassed, no energy passes through it, and the high-frequency transformer T is in an idle state.

[0044] The direct boost discharge mode bypasses the high-frequency transformer T, avoiding voltage spikes caused by the iron loss, copper loss, and leakage inductance of the high-frequency transformer T, resulting in higher system efficiency (based on actual use and data calculations, efficiency can reach over 97% at 200V input, 800V output, and 50kW). The mode switch M provides a low-impedance DC path, reducing losses compared to using diode freewheeling. The direct boost discharge mode, in conjunction with the enhanced boost discharge mode, achieves optimal efficiency across the entire input voltage range: at low voltages, the high-frequency transformer T is used to increase gain; at high voltages, it is bypassed to reduce losses. The control circuit detects the input voltage; for example, when the voltage rises from low to high above 55V, it switches to the direct boost discharge mode; when the voltage falls from high to low below 50V, it switches back to the enhanced boost discharge mode, with a hysteresis range of 5V, avoiding frequent switching.

[0045] To optimize the above technical solution, in the enhanced boost discharge mode, the voltage gain of the bidirectional DC-DC converter is made to meet the following requirements by adjusting the first duty cycle of the first switch S1, the second switch S2, the third switch S3, and the fourth switch S4. Where n is the turns ratio of the high-frequency transformer T. The first duty cycle is used; in direct boost discharge mode, by adjusting the second duty cycles of the first switch S1, the second switch S2, the third switch S3, and the fourth switch S4, the voltage gain of the converter is made to meet the requirements. ,in This is the second duty cycle.

[0046] Specifically, in direct boost discharge mode, the converter operates in a traditional Boost topology (input inductor L1, switching transistors (four transistors in parallel on the primary side of the full bridge are equivalent to one switch), diodes (body diode of the mode switching switch M) or synchronous rectifier diodes). The voltage gain formula is as follows: Where D2 is the second duty cycle, i.e., the duty cycle when the first switch S1, the second switch S2, the third switch S3, and the fourth switch S4 are simultaneously turned on. This embodiment clarifies the quantitative relationship between the duty cycle and voltage gain in two modes through formulas. Compared with traditional dual active bridge converters, this invention utilizes the dual effects of input inductor L1 and high-frequency transformer T to boost voltage at low input voltages, avoiding reactive circulating current caused by phase shift control; at high input voltages, it switches to Boost mode, with the duty cycle far from the limit value, improving control accuracy. Simultaneously, the control circuit can calculate the target duty cycle in real time based on the input voltage, achieving smooth switching, ensuring stable output voltage, and shortening dynamic response time.

[0047] To optimize the above technical solution, the control circuit is configured such that when the DC bus charges the flow battery and the input voltage of the flow battery is lower than the third preset threshold, the fifth switch Q1 and the sixth switch Q2 are controlled to conduct alternately at high frequency, and the first switch S1, the second switch S2, the third switch S3 and the fourth switch S4 are conducted in a synchronous rectification mode, so that the converter operates in the enhanced step-down charging mode, and the energy is stepped down from the DC bus by the high-frequency transformer T and then transferred to the flow battery through the input inductor L1.

[0048] Specifically, the control circuit performs the following operations: the mode switching switch M remains off; the fifth switch Q1 and the sixth switch Q2 of the secondary half-bridge are alternately turned on at high frequency, with a duty cycle denoted as... (The third duty cycle below), and <0.5 (usually taken as 0.2 to 0.4); the first switch S1, the second switch S2, the third switch S3 and the fourth switch S4 of the primary side full bridge are turned on in synchronous rectification mode.

[0049] Specifically, see Figure 6 and Figure 7 The enhanced buck charging mode comprises two phases within one switching cycle: Phase 2-A: Switch Q1 is on, switch Q2 is off; switches S1 and S4 are on (synchronous rectification), switches S2 and S3 are off, and mode switching switch M remains off. The current path is as follows: Secondary side: The current output from the positive terminal of the DC bus splits into two branches. The first branch flows through switch Q1 into the secondary winding of the high-frequency transformer T, and the second branch flows through capacitor C1 into the midpoint between capacitors C1 and C2. Both currents return to the negative terminal of the DC bus via capacitor C2. Primary side: Switches S1 and S4 are on. The current path is: primary winding of high-frequency transformer T → switch S1 → input inductor L1 → battery → switch S4 → primary winding of high-frequency transformer T. In this phase, the high-frequency transformer T transfers energy from the high-voltage side to the low-voltage side, and the input inductor L1 stores some energy, causing the current to rise.

[0050] Phase 2-B: Sixth switch Q2 is on, fifth switch Q1 is off, second switch S2 and third switch S3 are on (synchronous rectification), first switch S1 and fourth switch S4 are off, and mode switching switch M is off. The current path is as follows: Secondary side: The current output from the positive terminal of the DC bus splits into two branches after passing through the first capacitor C1. The first branch returns to the negative terminal of the DC bus via the secondary winding of the high-frequency transformer T and the sixth switch Q2; the second branch returns to the negative terminal of the DC bus via the second capacitor C2. Primary side: Second switch S2 and third switch S3 are on. The current path is: primary winding of high-frequency transformer T → third switch S3 → input inductor L1 → battery → second switch S2 → primary winding of high-frequency transformer T. Energy continues to be transferred; the input inductor L1 releases energy, and the current decreases linearly.

[0051] Third duty cycle This is the duty cycle of the fifth switch Q1 (or the sixth switch Q2). Because... A value less than 0.5, and n is typically greater than 1, can achieve a significant voltage reduction. For example, taking n=8, =0.4, then Vin = 0.4 / 8 × 800V = 40V, which meets the low-voltage charging requirements. Adjust the third duty cycle. The charging voltage can be changed. The enhanced buck charging mode achieves main voltage reduction through a high-frequency transformer T, while the input inductor L1 acts as a filter and energy buffer, avoiding the control difficulties of traditional Buck converters at extremely small duty cycles (D close to 0). The synchronous rectification scheme reduces the conduction losses of the primary-side full-bridge. Simultaneously, the continuous current characteristic of the input inductor L1 results in very small charging current ripple on the flow battery side, which is beneficial for battery protection. This mode is symmetrical to the enhanced boost discharge mode, realizing a bidirectional composite gain path.

[0052] To optimize the above technical solution, the control circuit is also configured such that when the input voltage of the flow battery is higher than the fourth preset threshold, the control mode switching switch M operates at high frequency and controls the first switch S1, the second switch S2, the third switch S3 and the fourth switch S4 to be turned on or off at the same time, so that the bidirectional DC-DC converter operates in the direct buck charging mode, and the energy is transferred from the DC bus to the flow battery through the mode switching switch M and the input inductor L1.

[0053] Specifically, the direct buck charging mode is suitable for situations where the DC bus charges the flow battery and the flow battery input voltage is higher than the fourth preset threshold (e.g., 55V, with hysteresis), meaning the battery is in a high state of charge and the input voltage is high (50V~200V). At this time, energy flows from the high-voltage DC bus (800V) to the flow battery, but because the battery voltage is high, there is no need for a high-frequency transformer T to significantly reduce the voltage. Buck converter consisting of mode switching switch M and input inductor L1 is sufficient to complete the buck charging.

[0054] Specifically, see Figure 8 and Figure 9 The direct buck charging mode consists of two phases within one switching cycle: Phase 4-A: Mode switching switch M is on, and switches S1, S2, S3, and S4 are all off, as are switches Q1 and Q2. Two current loops exist in this phase: Loop 1 (Battery charging main loop): Second capacitor C2, first capacitor C1, mode switching switch M, and input inductor L1. In this loop, input inductor L1 stores energy, the current increases linearly, and energy is simultaneously transferred from the DC bus side to the battery side via capacitive coupling. Loop 2 (Bus-side filter loop): Starting from the positive terminal of the DC bus, the current flows sequentially through the first capacitor C1 and the second capacitor C2, finally returning to the negative terminal of the DC bus. This loop maintains the voltage balance of the output capacitor unit, providing a smooth DC voltage. Energy exchange is achieved between the two loops through the first capacitor C1 and the second capacitor C2. During this phase, the high-frequency transformer T is in an open-circuit state due to the primary-side switches being all off and does not participate in operation.

[0055] Phase 4-B: Mode switching switch M is off. Switches S1, S2, S3, and S4 are turned on for synchronous rectification, while switches Q1 and Q2 remain off. Input inductor L1 continues to discharge to the flow battery. In this freewheeling circuit, the inductor current decreases linearly, and energy is continuously delivered to the flow battery. The high-frequency transformer T remains inactive.

[0056] To optimize the above technical solution, in the enhanced buck charging mode, the third duty cycle of the fifth switch Q1 and the sixth switch Q2 is adjusted to ensure that the voltage gain of the bidirectional DC-DC converter meets the requirements. Where n is the turns ratio of the high-frequency transformer T. The third duty cycle; in direct buck charging mode, the fourth duty cycle is adjusted by the mode switching switch M to ensure that the voltage gain of the bidirectional DC-DC converter meets the requirements. ,in This is the fourth duty cycle.

[0057] Specifically, the fourth duty cycle The duty cycle of the mode switching switch M. It directly determines the output voltage ratio. When required... When the gain is below 0.1, the bidirectional DC-DC converter automatically switches to the enhanced buck charging mode. This embodiment clarifies the quantitative relationship between the gain in the two modes of the buck direction through a formula, enabling the control system to accurately calculate the duty cycle and achieve rapid adjustment of the output voltage. Compared with traditional dual active bridge converters, it avoids reactive circulating current caused by phase shift control and improves light-load efficiency.

[0058] Secondly, embodiments of this application also provide a control method for using a bidirectional DC-DC converter as described above, comprising: detecting the input voltage or state of charge of a flow battery through a control circuit; and controlling the on / off state of a mode switching switch M and the operating states of the first switch S1, the second switch S2, the third switch S3, the fourth switch S4, the fifth switch Q1, and the sixth switch Q2 according to the detection result of the control circuit, so as to switch the operating mode of the bidirectional DC-DC converter.

[0059] To optimize the above technical solutions, the operating modes include enhanced boost discharge mode, enhanced buck charging mode, direct boost discharge mode, and direct buck charging mode; among them, enhanced boost discharge mode and direct boost discharge mode are operating modes in the discharge direction, and enhanced buck charging mode and direct buck charging mode are operating modes in the charging direction.

[0060] To optimize the above technical solution, when the bidirectional DC-DC converter operates in enhanced boost discharge mode, the control mode switching switch M is turned off, and the first switch S1 and the fourth switch S4 are controlled to be alternately turned on at high frequency with the second switch S2 and the third switch S3, so that the input inductor L1 and the high-frequency transformer T form a composite boost path; when the bidirectional DC-DC converter operates in direct boost discharge mode, the control mode switching switch M is turned off or its synchronous rectification is turned on, and the first switch S1, the second switch S2, the third switch S3 and the fourth switch S4 are controlled to be turned on or off simultaneously, so that the input inductor L1 is a boost energy storage element, and the mode switching switch M provides direct current from the input inductor L1 to the DC bus. Connection path; when the bidirectional DC-DC converter operates in enhanced buck charging mode, the fifth switch Q1 and the sixth switch Q2 are controlled to conduct alternately, and the first switch S1, the second switch S2, the third switch S3 and the fourth switch S4 are controlled to conduct in a synchronous rectification manner, so that the DC bus energy is stepped down by the high-frequency transformer T and then transmitted to the flow battery through the input inductor L1; when the bidirectional DC-DC converter operates in direct buck charging mode, the mode switching switch M is controlled to operate at high frequency, and the first switch S1, the second switch S2, the third switch S3 and the fourth switch S4 are controlled to conduct or be turned off simultaneously, so that the DC bus energy is directly transmitted to the flow battery through the mode switching switch M and the input inductor L1.

[0061] The specific structure of the bidirectional DC-DC converter is as described in the above embodiments. Since this control method adopts all the technical solutions of all the above embodiments, it has at least all the beneficial effects brought by the technical solutions of the above embodiments, which will not be described in detail here.

[0062] It should be noted that the bidirectional DC-DC converter and control method provided by this invention can be used in the field of piezoelectric conversion technology or other fields. Other fields refer to any field other than piezoelectric conversion technology. The above are merely examples and do not limit the application areas of the bidirectional DC-DC converter and control method provided by this invention.

[0063] It should be understood that the terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting. Unless the context clearly indicates otherwise, the singular forms “a,” “an,” and “described” as used herein may also include the plural forms. The terms “comprising,” “including,” “containing,” and “having” are inclusive and therefore indicate the presence of the stated features, steps, operations, elements, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, elements, components, and / or combinations thereof. The method steps, processes, and operations described herein are not construed as requiring them to be performed in a particular order described or illustrated unless the order of performance is explicitly indicated. It should also be understood that additional or alternative steps may be used.

[0064] Although terms such as first, second, third, etc., may be used in this document to describe multiple elements, components, regions, layers, and / or segments, these elements, components, regions, layers, and / or segments should not be limited by these terms. These terms may be used only to distinguish one element, component, region, layer, or segment from another. Unless the context clearly indicates otherwise, terms such as "first," "second," and other numerical terms used herein do not imply order or sequence. Therefore, the first element, component, region, layer, or segment discussed below may be referred to as the second element, component, region, layer, or segment without departing from the teachings of the exemplary embodiments.

[0065] The above description is merely a specific embodiment of this application, enabling those skilled in the art to understand or implement this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A bidirectional DC-DC converter, characterized in that, It includes an input inductor, a primary-side full-bridge circuit, a high-frequency transformer, a secondary-side half-bridge circuit, an output capacitor unit, a mode switching switch, and a control circuit, wherein: The first end of the input inductor is used to connect to the positive terminal of the flow battery; The primary-side full-bridge circuit consists of a first switch, a second switch, a third switch, and a fourth switch. The first switch and the second switch are connected in series to form a first bridge arm. The third switch and the fourth switch are connected in series to form a second bridge arm. The first bridge arm and the second bridge arm are connected in parallel. The primary-side full-bridge circuit has a positive input terminal and a negative input terminal. The negative input terminal of the primary-side full-bridge circuit is used to connect to the negative electrode of the flow battery. The two ends of the primary winding of the high-frequency transformer are respectively connected to the midpoint of the first bridge arm and the midpoint of the second bridge arm. The secondary half-bridge circuit consists of a fifth switch and a sixth switch connected in series; The output capacitor unit consists of a first capacitor and a second capacitor connected in series, and the two ends of the output capacitor unit are used to connect to the DC bus. The first terminal of the mode switching switch is connected to the second terminal of the input inductor, the second terminal of the mode switching switch is connected to the positive terminal of the output capacitor unit, and the positive input terminal of the primary-side full-bridge circuit is connected to the connection node between the input inductor and the mode switching switch. The secondary half-bridge circuit is connected in parallel with the output capacitor unit. The first end of the secondary winding of the high-frequency transformer is connected to the connection node of the fifth switch and the sixth switch, and the second end of the secondary winding is connected to the connection node of the first capacitor and the second capacitor. The control circuit is connected to the primary-side full-bridge circuit, the secondary-side half-bridge circuit, and the mode switching switch, respectively, and is used to control the on / off state of the mode switching switch and the working state of the first switch, the second switch, the third switch, the fourth switch, the fifth switch, and the sixth switch according to the input voltage or state of charge of the flow battery.

2. The bidirectional DC-DC converter according to claim 1, characterized in that, The control circuit is configured as follows: When the input voltage of the flow battery is lower than the first preset threshold, the mode switching switch is turned off, and the first and fourth switches are alternately turned on at high frequency in conjunction with the second and third switches. The bidirectional DC-DC converter operates in the enhanced boost discharge mode. The input inductor and the high-frequency transformer together form a composite boost path. Energy is boosted by the flow battery through the input inductor and then boosted by the high-frequency transformer to the DC bus.

3. The bidirectional DC-DC converter according to claim 2, characterized in that, The control circuit is also configured to: When the input voltage of the flow battery is higher than the second preset threshold, the mode switching switch is controlled to turn off or to be synchronously rectified and turned on. The first switch, the second switch, the third switch and the fourth switch are controlled to be turned on or off at the same time, so that the bidirectional DC-DC converter operates in the direct boost discharge mode. The input inductor is a boost energy storage element. The energy is boosted by the flow battery through the input inductor and then transmitted to the DC bus through the mode switching switch.

4. The bidirectional DC-DC converter according to claim 3, characterized in that, In the enhanced boost discharge mode, by adjusting the first duty cycle of the first switch, the second switch, the third switch, and the fourth switch, the voltage gain of the bidirectional DC-DC converter is made to satisfy... Where n is the turns ratio of the high-frequency transformer. This is the first duty cycle; In the direct boost discharge mode, by adjusting the second duty cycle of the first switch, the second switch, the third switch, and the fourth switch, the voltage gain of the converter is made to satisfy... ,in This is the second duty cycle.

5. The bidirectional DC-DC converter according to claim 1, characterized in that, The control circuit is configured as follows: When the DC bus charges the flow battery and the input voltage of the flow battery is lower than the third preset threshold, the fifth and sixth switches are controlled to conduct alternately at high frequency, and the first, second, third, and fourth switches are conducted in a synchronous rectification mode, so that the converter operates in the enhanced step-down charging mode. The energy is stepped down by the high-frequency transformer from the DC bus and then transferred to the flow battery through the input inductor.

6. The bidirectional DC-DC converter according to claim 5, characterized in that, The control circuit is also configured to: When the input voltage of the flow battery is higher than the fourth preset threshold, the mode switching switch is controlled to operate at high frequency, and the first switch, the second switch, the third switch and the fourth switch are controlled to be turned on or off at the same time, so that the bidirectional DC-DC converter operates in direct buck charging mode, and energy is transferred from the DC bus to the flow battery through the mode switching switch and the input inductor.

7. The bidirectional DC-DC converter according to claim 6, characterized in that, In the enhanced buck charging mode, by adjusting the third duty cycle of the fifth and sixth switches, the voltage gain of the bidirectional DC-DC converter is made to meet the requirements. Where n is the turns ratio of the high-frequency transformer. This refers to the third duty cycle; In the direct buck charging mode, by adjusting the fourth duty cycle of the mode switching switch, the voltage gain of the bidirectional DC-DC converter is made to meet the requirements. ,in This refers to the fourth duty cycle.

8. A control method, characterized in that, The application of the bidirectional DC-DC converter as described in any one of claims 1 to 7 includes: The input voltage or state of charge of the flow battery is detected by the control circuit. Based on the detection results of the control circuit, the on / off state of the control mode switching switch and the working states of the first, second, third, fourth, fifth, and sixth switches are controlled to switch the working mode of the bidirectional DC-DC converter.

9. The control method according to claim 8, characterized in that, The operating modes include enhanced boost discharge mode, enhanced buck charge mode, direct boost discharge mode, and direct buck charge mode; The enhanced boost discharge mode and the direct boost discharge mode are operating modes in the discharge direction, while the enhanced buck charging mode and the direct buck charging mode are operating modes in the charging direction.

10. The control method according to claim 9, characterized in that, When the bidirectional DC-DC converter operates in the enhanced boost discharge mode, the mode switching switch is turned off, and the first switch and the fourth switch are switched on alternately with the second switch and the third switch at complementary high frequencies, so that the input inductor and the high-frequency transformer form a composite boost path. When the bidirectional DC-DC converter operates in the direct boost discharge mode, the mode switching switch is controlled to turn off or to be synchronously rectified and turned on, and the first switch, the second switch, the third switch and the fourth switch are controlled to be turned on or off at the same time, so that the input inductor is a boost energy storage element, and the mode switching switch provides a direct path from the input inductor to the DC bus. When the bidirectional DC-DC converter operates in the enhanced buck charging mode, the fifth switch and the sixth switch are controlled to conduct alternately, and the first switch, the second switch, the third switch and the fourth switch are controlled to conduct in a synchronous rectification manner, so that the DC bus energy is stepped down by the high-frequency transformer and then transmitted to the flow battery through the input inductor; When the bidirectional DC-DC converter operates in the direct buck charging mode, the high-frequency switch of the mode switching switch is controlled to operate, and the first switch, the second switch, the third switch and the fourth switch are controlled to be turned on or off at the same time, so that the DC bus energy is directly transmitted to the flow battery through the mode switching switch and the input inductor.

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